Automotive Mass Air Flow Calculation

[firewalker]

Does anyone knows the formula for calculating the mass air flow in gr/sec a M.A.F. sensor should report in normal operation?

I know we need Engine Capacity (CC³), Engine Speed (RPM), Intake Absolute Pressure (mbar), Air Temperature (Celcius). There is this calculator

https://tfautomotive.co.uk/tech-assist/calculators/mass-air-flow-calculator

Testing various values I can see that the result is analogous of the above quantities. Doubling any of them doubles the results (for the temperature the doubling is for in Kelvin).

[HighVoltage]

This one worked for me:

MAF [g/s] = Mair ∙ Vd ∙ nvol ∙ (MAP – (patm/CR)) / (R ∙ IAT)

where:
Mair is molecular weight of air of 28.9 g.mol-1
Vd is engine displacement volume in dm3
nvol is dimensionless engine volumetric efficiency multiplier
MAP is intake manifold absolute pressure in kPa
patm is barometric pressure in kPa
CR is dimensionless engine compression ratio
R is universal gas constant of 8.314 J.mol-1.K-1
IAT is intake manifold charge temperature in K


http://archive.sciendo.com/MECDC/mecdc.2014.12.issue-1/mecdc-2014-0002/mecdc-2014-0002.pdf

[HighVoltage]

A rule of thumb is:

1 g/s at idle per cylinder or just below.

Depending on the engine, you can reach 250 g/s or 900 kg/h going through the MAFS at full load.

[firewalker]

I guess the use a standard value for n_vol and the CR depends if the engine is petrol or diesel.

Alexander.

[bdunham7]

I'm not sure what exactly you are trying to do, but estimating mass air flow so as to compare it to an actual measured flow has at least two significant obstacles that limit how close you can get.  One is that volumetric efficiency can vary quite a bit on some engines and the other is that some mass air flow sensors measure reversion additively and this has to be accounted for in the ECM design. 

[firewalker]

The calculators (like the one in my first post) can help a technician diagnose a malfunctioning MAF sensor. You compare the values when in idle. I was wondering hoe are they guessing the mass air flow.

Alexander.

[HighVoltage]

The malfunction of a MAFS can be of many aspects and not only the output signal. Some cars measure the supply current to the MAFS within a window and set a error code, if the current goes outside this window.

Many years ago I built a big MAFS test stand for a customer to find MAFS problems. And this showed the biggest problems exist during acceleration of the airflow during the first 25 to 50 ms of load change.